The use of electron beam irradiation to form a patterned resist structure on a substrate is known. The use of such resist structures for the further processing of the substrate, e.g. selective etching, metal deposition and the like, is likewise known. The increasing demand for very-large-scale-integrated (VLSI) devices has made imperative the formation of such structures having patterns on the order of one micrometer and below with accuracy and reproducibility.
The capability of electron beam generating equipment to produce an exceptionally narrow beam, e.g. 0.5 micrometer and below, is a significant advantage for fine-line lithography. It is recognized, however, that electron beam irradiation will backscatter from the substrate surface in much the same manner as light utilized to irradiate a photoresist will reflect from the substrate with a resultant loss of pattern resolution.
A second problem inherent in electron beam lithography is the build-up of a charge in the resist layer and on the substrate surface during irradiation. The deposited charge can cause aiming errors and pattern misalignment, both in orientation on the substrate surface and in the actual writing of the pattern.
The backscattering of an electron beam, commonly referred to as the proximity effect, has been addressed, for example, by varying the local irradiation, i.e. the scan rate and the dwell time, of the beam. While this is a reasonable approach for low-powered apparatus, variance of local irradiation is, at best, limited as the capacity of such apparatus approaches 300 MHz.
An approach to solving the problem of charge build-up is the deposition of a thin conductive coating on the substrate, or on the resist itself, as described in U.S. Pat. No. 3,893,127, issued July 1, 1975. This film, which is preferably 10-100 nm thick, is of a conductive metal, e.g. copper, aluminum, nickel and the like, or a layer of glass with a thin coating of a conductive oxide such as tin oxide or indium oxide. The use of such coatings is disadvantageous in that they require extra process steps for their patterning and subsequent removal.
U.S. Pat. No. 4,456,677, issued June 26, 1984, discloses a composite resist structure wherein a high atomic number film, e.g. gold, overlies a low atomic number film, e.g. aluminum, which in turn overlies a layer of positive resist. The thickness of the two films is selected so that only a small percentage of the electrons striking the top layer actually passes into the resist film. It is also stated that a single layer of resist impregnated with a high atomic number material could be utilized. In either instance, the high atomic number material will absorb a substantial portion of the electron energy, thus requiring the use of a highly sensitive resist material.
Another proposed electron beam resist medium is a trilayer structure comprising a base or planarizing layer and an electron beam resist layer separated by a thin layer, e.g. 60-100 nm thick, of silicon. In such a structure, the bottom layer must be sufficiently thick to prevent proximity effect and the silicon layer sufficiently thin so that it does not cause backscattering of the electron beam, yet sufficiently thick to dissipate charge build-up. It will be appreciated that, in addition to the extra process steps required for a three layer structure, both of the structures described above require that at least one of the layers meet very exacting criteria for thickness and uniformity.
A simplified resist structure for electron beam lithography is provided in accordance with this invention.